linux-kernel - [PATCH 2/2] kvm: mmu: don't do overflow memslot check

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Date:	Tue, 14 Apr 2015 13:51:30 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Eric Northup <digitaleric@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Wanpeng Li <wanpeng.li@...ux.intel.com>
Subject: [PATCH 2/2] kvm: mmu: don't do overflow memslot check

As Andre pointed out:

| I don't understand the value of this check here. Are we looking for a
| broken memslot? Shouldn't this be a BUG_ON? Is this the place to care
| about these things? npages is capped to KVM_MEM_MAX_NR_PAGES, i.e.
| 2^31. A 64 bit overflow would be caused by a gigantic gfn_start which
| would be trouble in many other ways.

This patch drops the memslot overflow check to make the codes more simple.

Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
---
 arch/x86/kvm/mmu.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 2a0d77e..9265fda 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4505,19 +4505,12 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
 	bool flush = false;
 	unsigned long *rmapp;
 	unsigned long last_index, index;
-	gfn_t gfn_start, gfn_end;
 
 	spin_lock(&kvm->mmu_lock);
 
-	gfn_start = memslot->base_gfn;
-	gfn_end = memslot->base_gfn + memslot->npages - 1;
-
-	if (gfn_start >= gfn_end)
-		goto out;
-
 	rmapp = memslot->arch.rmap[0];
-	last_index = gfn_to_index(gfn_end, memslot->base_gfn,
-					PT_PAGE_TABLE_LEVEL);
+	last_index = gfn_to_index(memslot->base_gfn + memslot->npages - 1,
+				memslot->base_gfn, PT_PAGE_TABLE_LEVEL);
 
 	for (index = 0; index <= last_index; ++index, ++rmapp) {
 		if (*rmapp)
@@ -4535,7 +4528,6 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
 	if (flush)
 		kvm_flush_remote_tlbs(kvm);
 
-out:
 	spin_unlock(&kvm->mmu_lock);
 }
 
-- 
1.9.1

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